動態加載與插件系統的初步實現（三）：WinForm示例

代碼文件在此Download，本文章圍繞前文所述默認AppDomain、插件容器AppDomain兩個域及IPlugin、PluginProvider、PluginProxy3個類的使用與變化進行。

添加WinForm項目Host、類庫Plugin、引用System.Windows.Forms;的類庫Plugin_A與Plugin_B，其中Plugin_A、Plugin_B的項目屬性中，“生成”選項卡中“輸出路徑”設置為..\Host\bin\Debug\，即指向Host項目的Bin目錄。

考慮到WinForm項目常常涉及多級菜單構建，這里以兩級菜單示例。

Plugin項目中IPlugin代碼：

public interface IPlugin
{
    IList<String> GetMenus();
    IList<String> GetMenus(String menu);
    void Notify(Object userState);
}

其中無參方法GetMenus()提取一級菜單，有參重載GetMenus(String menu)提取二級菜單，Notify(Object userState)是兩個應用程序域的通知調用。

PluginProxy繼承MarshalByRefObject，代碼長點：

public class PluginProxy : MarshalByRefObject, IDisposable
{
    private readonly static PluginProxy instance = new PluginProxy();

    public static PluginProxy Instance
    {
        get { return instance; }
    }

    private PluginProxy()
    {
    }

    private AppDomain hostDomain = null;
    private PluginProvider proxy = null;

    public PluginProvider Proxy
    {
        get
        {
            if (hostDomain == null)
            {
                hostDomain = AppDomain.CreateDomain("PluginHost");
            }
            if (proxy == null)
            {
                Type proxyType = typeof(PluginProvider);
                proxy = (PluginProvider)hostDomain.CreateInstanceAndUnwrap(proxyType.Assembly.FullName, proxyType.FullName);
            }
            return proxy;
        }
    }

    public void Unload()
    {
        if (hostDomain != null)
        {
            proxy = null;
            AppDomain.Unload(hostDomain);
            hostDomain = null;
        }
    }

    public void Dispose()
    {
        Unload();
    }
}

PluginProvider除構造函數外，Notify(IPlugin plugin, Object userState)方法將調用IPlugin插件的Notify方法：

public class PluginProvider : MarshalByRefObject
{
    [ImportMany]
    public IEnumerable<Lazy<IPlugin>> Plugins { get; set; }

    public PluginProvider()
    {
        AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new DirectoryCatalog("."));
        CompositionContainer container = new CompositionContainer(catalog);
        container.ComposeParts(this);
    }

    public void Notify(IPlugin plugin, Object userState)
    {
        plugin.Notify(userState);
    }
}

然后是插件Plugin_A、Plugin_B的實現。添加Plugin類（類名與命名空間隨意）引用System.ComponentModel.Composition，加入[Export(typeof(IPlugin))]修飾。這里使用了一份XML顯示菜單目錄，將在得到通知后將一個Form彈出來：

[Export(typeof(IPlugin))]
public class PluginA : MarshalByRefObject, IPlugin
{
    private String menus =
        @"<Component>
            <Net>
              <AuthenticationManager />
              <Authorization />
              <Cookie />
            </Net>
            <IO>
              <ErrorEventArgs />
              <FileSystemEventArgs />
            </IO>
        </Component>";

    public IList<String> GetMenus()
    {
        return XElement.Parse(menus).Elements().Select(x => x.Name.LocalName).ToArray();
    }

    public IList<String> GetMenus(String menu)
    {
        return XElement.Parse(menus).Elements(menu).Elements().Select(x => x.Name.LocalName).ToArray();
    }

    public void Notify(Object userState)
    {
        String text = (String)userState;
        Label label = new Label()
        {
            Text = text,
            AutoSize = false,
            Dock = DockStyle.Fill,
            TextAlign = System.Drawing.ContentAlignment.MiddleCenter,
        };
        Form frm = new Form();
        frm.Controls.Add(label);
        frm.ShowDialog();
    }
}

Plugin_B與Plugin_A類似，不再重復，然后是Host實現。Host使用了兩個FlowLayoutPanel分別用於顯示一級菜單與兩級菜單。

Load按鈕加載插件列表，將每個插件綁定到一個Button上：

private void button1_Click(object sender, EventArgs e)
{
    flowLayoutPanel1.Controls.Clear();
    textBox1.AppendText("PluginProvider loaded");
    textBox1.AppendText(Environment.NewLine);

    PluginProvider proxy = PluginProxy.Instance.Proxy;

    IEnumerable<Lazy<IPlugin>> plugins = proxy.Plugins;
    foreach (var plugin in plugins)
    {
        foreach (var menu in plugin.Value.GetMenus())
        {
            Button menuBtn = new Button();
            menuBtn.Text = menu;
            menuBtn.Tag = plugin.Value;
            menuBtn.Click += menuBtn_Click;
            flowLayoutPanel1.Controls.Add(menuBtn);
        }
    }
}

private void menuBtn_Click(object sender, EventArgs e)
{
    flowLayoutPanel2.Controls.Clear();
    Button menuBtn = (Button)sender;

    try
    {
        IPlugin plugin = (IPlugin)menuBtn.Tag;
        foreach (var item in plugin.GetMenus(menuBtn.Text))
        {
            Button itemBtn = new Button();
            itemBtn.Text = item;
            itemBtn.Tag = plugin;
            itemBtn.Click += itemBtn_Click;
            flowLayoutPanel2.Controls.Add(itemBtn);
        }
    }
    catch (AppDomainUnloadedException)
    {
        textBox1.AppendText("Plugin domain have been uloaded");
        textBox1.AppendText(Environment.NewLine);
    }
}

private void itemBtn_Click(object sender, EventArgs e)
{
    try
    {
        Button menuBtn = (Button)sender;
        IPlugin plugin = (IPlugin)menuBtn.Tag;
        PluginProvider proxy = PluginProxy.Instance.Proxy;
        proxy.Notify(plugin, menuBtn.Text);
    }
    catch (AppDomainUnloadedException)
    {
        textBox1.AppendText("Plugin domain not loaded");
        textBox1.AppendText(Environment.NewLine);
    }
}

Unload按鈕卸載插件AppDomain：

private void button2_Click(object sender, EventArgs e)
{
    PluginProxy.Instance.Unload();
    textBox1.AppendText("PluginProvider unloaded");
    textBox1.AppendText(Environment.NewLine);
}

Delete按鈕移除Plugin_A.dll、Plugin_B.dll：

private void button3_Click(object sender, EventArgs e)
{
    try
    {
        String[] pluginPaths = new[] { "Plugin_A.dll", "Plugin_B.dll" };
        foreach (var item in pluginPaths)
        {
            if (System.IO.File.Exists(item))
            {
                System.IO.File.Delete(item);
                textBox1.AppendText(item + " deleted");
            }
            else
            {
                textBox1.AppendText(item + " not exist");
            }
            textBox1.AppendText(Environment.NewLine);
        }
    }
    catch (Exception ex)
    {
        textBox1.AppendText(ex.Message);
        textBox1.AppendText(Environment.NewLine);
    }
}

運行結果如下：

我嘗試比較IEnumerable<Lazy<IPlugin>>與IEnumerable<IPlugin>的進程內存占用，在一個額外的Button里進行100加載與卸載，統計內存變化圖如下，有興趣的可以下載EXCEL文件看看：

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